Trait-mediated responses of caterpillar communities to spongy moth outbreaks and subsequent tebufenozide treatments.

Outbreaks of the spongy moth Lymantria dispar can have devastating impacts on forest resources and ecosystems. Lepidoptera-specific insecticides, such as Bacillus thuringiensis var. kurstaki (BTK) and tebufenozide, are often deployed to prevent heavy defoliation of the forest canopy. While it has been suggested that using BTK poses less risk to non-target Lepidoptera than leaving an outbreak untreated, in situ testing of this assumption has been impeded by methodological challenges. The trade-offs between insecticide use and outbreaks have yet to be addressed for tebufenozide, which is believed to have stronger side effects than BTK. We investigated the short-term trade-offs between tebufenozide treatments and no-action strategies for the non-target herbivore community in forest canopies. Over 3 years, Lepidoptera and Symphyta larvae were sampled by canopy fogging in 48 oak stands in southeast Germany during and after a spongy moth outbreak. Half of the sites were treated with tebufenozide and changes in canopy cover were monitored. We contrasted the impacts of tebufenozide and defoliator outbreaks on the abundance, diversity, and functional structure of chewing herbivore communities. Tebufenozide treatments strongly reduced Lepidoptera up to 6 weeks after spraying. Populations gradually converged back to control levels after 2 years. Shelter-building species dominated caterpillar assemblages in treated plots in the post-spray weeks, while flight-dimorphic species were slow to recover and remained underrepresented in treated stands 2 years post-treatment. Spongy moth outbreaks had minor effects on leaf chewer communities. Summer Lepidoptera decreased only when severe defoliation occurred, whereas Symphyta declined 1 year after defoliation. Polyphagous species with only partial host plant overlap with the spongy moth were absent from heavily defoliated sites, suggesting greater sensitivity of generalists to defoliation-induced plant responses. These results demonstrate that both tebufenozide treatments and spongy moth outbreaks alter canopy herbivore communities. Tebufenozide had a stronger and longer lasting impact, but it was restricted to Lepidoptera, whereas the outbreak affected both Lepidoptera and Symphyta. These results are tied to the fact that only half of the outbreak sites experienced severe defoliation. This highlights the limited accuracy of current defoliation forecast methods, which are used as the basis for the decision to spray insecticides.

Side Effects of Insecticides on Leaf-Miners and Gall-Inducers Depend on Species Ecological Traits and Competition with Leaf-Chewers.

Internal feeding is considered to shield sessile herbivorous insects from exposure to nonsystemic insecticides aerially sprayed against forest defoliators, although this has not been tested. It is, however, established that leaf damage caused by defoliators affects the survivorship and oviposition behavior of sessile herbivores. Thus feeding ecology and competition may mediate nontarget effects of insecticides on these insects. We tested the ecological sensitivity of 3 guilds of sessile herbivores (upper-surface leaf-miners, lower-surface leaf-miners, and gall-inducers) to the lipophilic larvicides diflubenzuron and tebufenozide aerially applied either at operational rates (12 g active ingredient [a.i.]/ha and 69.6 g [a.i.]/ha, respectively) or at maximum legal rates (60 g [a.i.]/ha and 180 g [a.i.]/ha, respectively), in German oak forests. Diflubenzuron affected leaf-miners at different life stages depending on their position on the leaf but had no effect on gall-inducers. Tebufenozide showed a similar, but not significant, pattern in leaf-miners and did not affect gall-inducers. By reducing the incidence of chewing damage on leaves, both insecticides offset the negative effect of competition on leaf-miner and gall-inducers. The net outcome of insecticide treatment was positive for guilds avoiding exposure, but negative for upper-surface leaf-miners. Exposure to insecticides in situ can be mediated by subtle differences in species biology and species interactions, with potential implications for organisms usually considered safe in risk assessment studies. Environ Toxicol Chem 2021;00:1-17. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Assessing Insecticide Effects in Forests: A Tree-Level Approach Using Unmanned Aerial Vehicles.

Large-scale field studies on the ecological effects of aerial forest spraying often face methodological challenges, such as insufficient funding, difficult logistics, and legal obstacles. The resulting routine use of underpowered designs could lead to a systematic underestimation of insecticide effects on nontarget arthropod communities. We tested the use of an Unmanned Aerial Vehicles (UAVs) for experimental insecticide applications at tree level to increase replication in cost-efficient way. We assessed the effects of two forestry insecticides, diflubenzuron (DFB) and tebufenozide (TBF), on the oak defoliator, Thaumetopoea processionea (Linnaeus) (Lepidoptera: Thaumetopoeidae), and on nontarget, tree-living Lepidoptera. Individual trees were sprayed with either insecticide or left unsprayed, in a fully factorial design involving 60 trees. Caterpillars fallen from tree crowns were sampled as a measure of mortality, while caterpillar feeding activity was monitored by collecting frass droppings. Both DFB and TBF led to greater mortality of T. processionea and lower Lepidoptera feeding activity than control levels. TBF caused measurable mortality in nontarget groups, affecting Macrolepidoptera more strongly than Microlepidoptera, while there was no significant side effect of DFB. The high treatment efficacy against the target pest indicates that UAV technology is well-suited for the application of insecticide in forests. We detected distinct responses to different insecticides among nontarget groups and suggest there is an influence of application timing and biological traits in these differences, emphasizing the need for more ecologically orientated risk assessment. UAV-supported designs can be used to link laboratory bioassays and large-scale experiments, allowing for more comprehensive assessments of insecticide effects in forest ecosystems.